1. Field of the Invention:
The present invention relates to a tensioner device for use in a chain drive mechanism to apply a proper tension to a chain of the chain drive mechanism.
2. Description of the Related Art:
Tensioner devices are used to exert a predetermined tension on a chain, a belt or a like power transmitting means. A typical example of the known tensioner devices is a ratchet tensioner device which is generally used in combination with a camshaft drive chain of a vehicle engine. The ratchet tensioner device includes a plunger slidably received in a housing. In order to apply a predetermined tension to the chain, the plunger partly projects from the housing. The plunger received in the housing defines jointly with the housing an oil chamber in which oil is supplied at a predetermined pressure while the engine is running. A ratchet mounted to the housing is held in mesh with ratchet teeth formed on an outer peripheral surface of the plunger along an axis thereof, so that retracting or backward movement of the plunger is prevented.
Another example of known tensioner devices is an oil-filled tensioner device which is particularly suitable for use with a belt having a small elongation. The oil-filled tensioner device includes a piston slidably received in a housing such that a high-pressure chamber and a low-pressure chamber separated by the piston are formed within the housing. The high-pressure chamber and the low-pressure chamber are filled with oil. In one direction from the low-pressure chamber to the high-pressure chamber, the oil can flow without substantive restriction. In the opposite direction from the high-pressure chamber to the low-pressure chamber, only a restricted flow of oil is permitted. The piston is urged by a spring in a direction to project from the housing.
In the ratchet tensioner device, the ratchet permits forward (advancing) movement of the plunger while preventing backward (retracting) movement of the plunger. The ratchet has a certain degree of backlash as it is held in mesh with the ratchet teeth on the plunger. The backlash must be kept small, or the chain tends to cause whipping. However, extremely small backlash hinders sufficient absorption of an increase in chain tension by the tensioner device. Thus, the ratchet tensioner device has a relatively low response to a change in the chain tension.
The oil-filled tensioner device has an excellent response to an increase and a decrease of the chain tension, but the stroke of the piston is relatively small. When used with a power transmission mechanism including a chain the elongation of which increases with time, the oil-filled tensioner device is made large in size to secure a large piston stroke and hence requires large space for installation thereof.